Were they synchros brand new? I don't see much where of the synchro teeth, but it can be one cause of gear pop out if they lose their taper that locks them into place. Of course if the shift pads are worn/missing or you have shifter cable issues, they won't engage all the way and it can cause premature wear on the teeth.

Same goes for the fibride material, if you have a clutch release issue (may not be noticeable to driver) they can wear faster.

Was it a new trans or remanned? By who? A buddy of mine bought a "reman" from a Chrysler dealer and within 2 months it grinded 3rd gear. Fibride completely worn. Since he opened it to install phantom grip, it voided warranty so I changed it. Shouldn't have failed that quickly, my guess is they "remanned" it but didn't replace every wear item so it failed early.